The goal of this study is to identify and investigate the mechanisms which determine the level at which body termperature is regulated and to determine how they are altered during fever. Fever produced by intrahypothalamic injections of prostaglandin E1 will be used as a tool. By observing and quantifying the alterations in thermoregulatory responses to PGE1 fever by calorimetry, we hope to achieve two major results. 1) An understanding of how peripheral and central elements of the thermoregulatory system are organized to maintain a normal body temperature and how pyrogens affect this organization to produce fever. b) Information on how pyrogens and other noxious stimuli act on the CNS to raise the level at which the physiological thermoregulator operates - fever. Several hypotheses are outlined together with the protocols to test their validity. These are: 1. the action of PGE1 fever on "hypothalamic setpoint". 2. The mechanisms which determine normal body temperature in afebrile animals. 3. The role of endogenously released prostaglandins in fever. The application of techniques, methods and equipment developed in previous studies are outlined and discussed.